The present invention relates to an image forming apparatus capable of setting up an adequate positional relation between images of different colors to be superposed on each other on a paper sheet and thereby insuring an attractive color image free from color deviation or irregular density.
A copier, printer or similar image forming apparatus includes various structural parts assembled together. Therefore, assembly errors between the parts and drive errors are apt to cause an image to be formed on a paper sheet at a position other than expected one. Particularly, in a full-color image that is a laminate of toner images of different colors, positional deviation between the images appears as conspicuous color deviation and degrades image quality to a critical degree. Moreover, to form a full-color image, a plurality of image carriers each are assigned to a particular color and therefore aggravate color deviation. A solution to this problem is extremely difficult to achieve.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-35288, 11-327416 and 4-190255 (Japanese Patent No. 2,889,368).
It is therefore an object of the present invention to provide an image forming apparatus capable of setting up, when images of different colors are superposed, an accurate positional relation between separate structural members and therefore an adequate positional relation between the images.
An image forming apparatus of the present invention includes an apparatus body to which a conveying unit is removably mounted. A belt is used to convey a paper sheet. A sensor includes a light emitting portion for emitting light toward the belt and a photosensitive portion to which the light from the belt is incident. One of the light emitting portion and photosensitive portion is mounted on the conveying unit while the other is mounted on the apparatus body. A positioning device causes, in interlocked relation to the mounting operation of the conveying unit to the apparatus body, the optical portion mounted on the apparatus body to move in accordance with the position and/or the configuration of the conveying unit and be positioned relative to the belt.